1. Field of the Invention
This invention relates to a partial cross-coupling reflector for use in quasi-optical millimeter wave power sources, and more specifically to a variable reflector that can select the amount of reflected power in both the co-polarized (co-pol) and cross-polarized (x-pol) fields.
2. Description of the Related Art
Power is difficult to produce at millimeter wave frequencies due to the lower power output of transistors and the losses incurred by traditional power combiners at these frequencies. Free space combining, also called “quasi optical” combining, eliminates the latter problem by allowing electromagnetic energy to combine in free space. Quasi optical arrays can provide high power by combining the outputs of many (e.g. thousands) of elements.
Quasi optical amplifiers arranged in arrays have been developed by a number of groups to produce high output powers at millimeter wave frequencies. These amplifier arrays amplify incoming radiation, either through reflection or transmission, and reradiate energy typically in a (more or less) gaussian mode. The amplifiers ususally utilize crossed input and output polarizations in order to reduce input/output coupling and avoid oscillation.
Quasi optical sources (oscillators) arranged in arrays have also been developed for millimeter wave power, and consist of a number of individual oscillators that are coupled together so that they mutually synchronize in phase and the radiation from all the elements combines coherently, typically in a (more or less) gaussian mode in front of the oscillator array. A number of different methods exist to realize the coupling network, from printed circuit transmission lines to partial reflectors. The key is to provide strong coupling between elements to ensure in-phase oscillation.
Many quasi optical oscillator arrays utilize hardwire circuitry (e.g. printed circuits, waveguides) to couple together the oscillating elements. For these types of arrays it is very difficult to control or modify the coupling in real time, without resorting to complicated schemes that are difficult to realize. For quasi optical arrays that utilize cavity resonators, the oscillators are usually one port devices (negative resistance oscillators) with a single polarization output, which increases parasitic mutual coupling, creating difficulty in controlling the coupling between elements.
A cavity resonator is typically realized using a total reflector and a partial reflector spaced a distance apart. Multiple reflections between the two reflectors creates standing waves at discrete resonant frequencies. The purpose of the partial reflector is to allow useful power to flow out of the structure. A typical partial reflector consists of a single grating. If the grating is is aligned with the polarization of an incident plane wave, the co-pol field will be reflected. By rotating the grating, specific amounts of the co-pol or x-pol field can be reflected. However, the other component of the reflected field is not controlled. In typical wave sources, one wants to control either the co- or x-pol component while nulling the other component to zero. Therefore the uncontrolled component is dissipated as energy, which makes the source less efficient.